Currently, the technological development and industrial applications of pretreatment, treatment and etching baths of polymers, mainly ABS and its mixtures such as ABS-Polycarbonate, Polyamide, polypropylene and thermoset polymers, with or without the addition of inorganic fillers and other additives in the mass of the polymer are based on the use of aqueous baths that contain chromium salts in oxidation state VI. The baths generally contain sulfuric acid and a chromium (VI) compound, mainly chromic acid, chromium trioxide or dichromates in a concentration of up to 400 gr/l.
This process of surface treatment or etching is the initial key treatment and a system of multiple functional layers of activation, acceleration, deposition of chemical nickel, copper, electrodeposition of copper, nickel and even final chrome plated finish, with their corresponding intermediate washes are then applied, thus obtaining a piece of plastic that has been metallized on its surface.
In the case of organic or painted coatings, after the initial key process of etching surface treatment described above, the plastic is coated or painted using one of the different methods that currently exist in the state of the art.
Only the combination of a suitable pre-treatment or etching, and several optimal subsequent treatments provide all of the necessary key requirements for the respective required applications, with the initial etching treatment being the fundamental factor of the whole process.
The chromium salts in oxidation state VI used in etching have been shown to be toxic and carcinogenic to humans, which generated interest in the substitution of the use of said chromium (VI) salts with any use of these salts and specifically for their use in polymer etching and treatment baths.
Different options have been proposed for the replacement of baths containing Cr(VI) salts with other chemical oxidants, mainly Mn (VII) salts, Mn (VI) salts and even Mn (III) salts, both in an acid medium and in a basic medium, as shown in patents U.S. Pat. No. 8,603,352 B1, US20130186774.
However, these systems based on Mn salts do not produce the same adhesion results in the subsequent metallization or coating of the polymer pieces and require very complex washes due to the need to eliminate the products of the reduction of the Mn salts, with the baths also being unstable over time and with short duration for industrial applications.
Alternatively, systems based on hydrogen peroxide as in DE19740431 C1, iron and/or copper salts and peroxides as in U.S. Pat. No. 6,559,242 B1, anionic complexes with transition metal chloride ligand U.S. Pat. No. 4,568,571 A, organometallic complexes of transition metals in organic solvent EP0081129 A1, salts of transition metal chlorides or nitrates without the use of complexes in slightly acidic aqueous medium US20070099425 A1, have been proposed.
In no case has it currently been possible to obtain results applicable to the final adhesion requirements that are demanded by the automotive sector among others and that are achieved through treatments based on Cr(VI) salts. A review article was recently published on this subject: “Plating on acrylonitrile—butadiene—styrene (ABS) plastic: a review Journal of Materials Science 2016, 51, 3657-3674”, Therefore, there is a need in the state of the art to have etching baths that are efficient in terms of adhesion results of metallization or coatings of other types on the polymer, stability of the bath over time and use, safety characteristics, non-toxicity for people and for the environment, ease of recovery, recycling or disposal under environmentally appropriate conditions of said treatment baths and free of Cr(VI).